Well... we Have Windows 2000 Server with IIS running. I have just install'ed 4.12 version of PHP and pointed other SMTP server in Our domain and defined FROM: address... It works fine. Just, I'm curious, how to define differnet FROM addreses for separate acounts.
We did not install sendmail or other mail server at all (or any utility to send mail).
I think, If You have any other SMTP server running, which You use for your mail to send in same domain (or other - but there may be rely problems), then the change of few lines in php.ini and server restart shoud work.
P.S. Sorry for my English